A2 Daily A Level Physics question
In a clean vacuum photocell illuminated by monochromatic light, the measured stopping potential is exactly zero. Which statement must be true?
Answer
The correct answer is A.
Correct: A — The light frequency equals the metal’s threshold frequency. A zero stopping potential means the fastest emitted electrons have zero kinetic energy at the surface, which occurs exactly at the threshold frequency (photon energy just equals the work function). A — This corresponds to zero maximum kinetic energy, so no retarding voltage is needed. B — Intensity controls the rate of emission, not the energy per electron, so it does not set the stopping potential. C — Electrode spacing does not determine the required stopping potential; it depends on the electrons’ maximum kinetic energy, not geometry. D — Below threshold there is no photoemission, so the stopping potential is not defined rather than being exactly zero.